1. Technical Field
The present disclosure relates to exhaust gas recirculation (EGR) systems having EGR coolers and preventing fouling of the EGR cooler by bypassing flow around the EGR cooler.
2. Background Art
One known approach to reduce the amount of NOx produced during combustion in an internal combustion engine is to mix in exhaust gases with the fresh air, commonly called exhaust gas recirculation (EGR). In diesel engines, very high levels of EGR can be tolerated. NOx is further reduced when EGR gases are cooled in the EGR loop, as NOx formation is highly sensitive to temperature. EGR cooling also reduces boost required as the EGR gases are more dense. Thus, an EGR cooler (or heat exchanger) is commonly disposed in the EGR duct.
Deposits form on the interior surfaces of the EGR cooler, first causing the EGR cooler to be less efficient and finally leading to plugging of the EGR cooler. To address that problem, EGR catalysts/filters have been provided in the EGR duct upstream of the EGR cooler. In some prior art systems, a catalyst is employed to oxidize unburned fuel and some particulate matter in the exhaust gases. In other prior art systems, a particulate filter is employed to remove the particulate matter from the exhaust gases. The requirement of a catalyst and/or filter in the EGR duct presents an additional cost and additional system complexity. In addition, EGR catalysts/filters provide a flow restriction that may adversely impact the available EGR flow rate.
Prior art engine control strategies may also control an EGR cooler bypass valve to partially or completely redirect EGR flow around the EGR cooler when exhaust gas temperature is below a threshold to reduce or eliminate formation of water condensation or to maintain charge temperatures in the intake manifold to a desired level at low speeds and loads. However, the prior art fails to recognize other conditions that contribute to accelerated fouling or plugging of an EGR cooler, particularly those associated with fuel condensation.